This present disclosure relates to x-ray imaging systems and methods, and more particularly the present disclosure relates to systems and methods of resetting photoconductive blocking type imaging detectors and x-ray light valve based imaging detectors.
Solid state photodetectors are used to detect light or other forms of radiation by converting the radiation to electric charge carriers in the form of electron-hole pairs. An electric potential is then applied to suitable electrodes on the detector, causing the charge to drift towards the electrodes from the point of excitation. Amorphous selenium (a-Se) has been used as a photoconductor in many applications including photocopiers, medical imaging systems and high-definition television broadcasting cameras. Because of its advantages over conventional semiconductors such as silicon, it has the potential for use in many advanced applications that involve photodetection.
While some photoconductive detectors provide a current when biased and illuminated, one class of photoconductive radiation detectors employs a blocking layer to trap photo-excited charges near or at an interface between the photoconductor and an insulating layer. After absorbing radiation in a photoconductive layer and collecting and trapping a sufficient amount of charge, the trapped charge may be interrogated to infer the power, intensity, or fluence of the radiation. For example, the trapped charge may be read out electrically or optically, and subsequently processed to determine an image.
One such photoconductive imaging device is the x-ray light valve (XLV), described in U.S. Pat. No. 7,687,792, which provides an electro-optic material (such as a liquid crystal layer) in contact with a photoconductive layer to convert the trapped charge pattern into spatially dependent anisotropy within the electro-optic material, which can in turn be optically interrogated by a readout optical beam. In general, XLVs include a photoconductor for charge generation and transport, a liquid crystal cell for image formulation, and a scanner for digital image readout. XLVs operate by absorbing x-rays in a photoconductor layer to generate local electrostatic charges. This charge builds up at an interface between the photoconductor and a liquid crystal cell, thereby changing the optical properties of the liquid crystal cell.
This scanning approach requires that the optical image be stable without significant decay over a pre-selected period of time (usually on the order of a few minutes) at the photoconductor-modulator interface. Because of the requirement of long lifetime, it becomes important to neutralize the remaining charge at the interface before a new exposure can be made.
Other blocking type photoconductive imaging devices are disclosed in U.S. Pat. Nos. 5,017,989, 5,510,626, 5,869,837 and 6,760,405. U.S. Pat. Nos. 5,017,989, 5,869,837 and 6,760,405 disclose devices in which the signal obtained by a photoconductor (or a photodiode) is capacitively coupled to a readout circuit, while U.S. Pat. No. 5,510,626 teaches a device in which a pixel-sized beam of readout radiation is raster scanned to produce charges that discharge the pixels and provide a readout current.
One significant drawback of blocking-type photoconductive imaging devices is that they must be reset between uses in order to attempt to neutralize the charges trapped at the interface of the photoconductor and the insulating layer. Such charges, if allowed to persist, affect the quality of subsequently detected images, leading to high background, blurring, and low signal-to-noise ratios.
Various methods of resetting blocking-type photoconductive imaging devices have been proposed. One known solution suggested in U.S. Pat. No. 7,687,792 involves illuminating the device with unfiltered white light while shorting the electrodes. Unfortunately, this method typically provides an imperfect neutralization of the residual charge and often merely smears the charge distribution present in the device without providing significant charge neutralization. After illuminating the photoconductive layer and producing electron-hole pairs, some of the trapped charges are neutralized in the photoconductor by recombining with carriers from the excited pairs. However, after a recombination process, one of the members of a given charge pair will still be present, and the dominant transport mechanism of diffusive transport will often result in the retrapping of the charge.
U.S. Pat. Nos. 5,017,989 and 7,687,792 suggest an improved method in which the device is reverse biased under illumination, where the illumination produces electron-hole pairs in the photoconductive layer that drift under the applied field and recombine with trapped charges at the interface. Unfortunately, this method still results in inefficient resetting, particularly in cases in which deep trap states exist at the photoconductor-insulator interface. The inefficient resetting of the device generates imaging artifacts that persist in reducing device performance when performing subsequent imaging.